xen-tools -- Unrestricted qemu logging

The Xen Project reports:

When the libxl toolstack launches qemu for HVM guests, it pipes the
output of stderr to a file in /var/log/xen. This output is not
rate-limited in any way. The guest can easily cause qemu to print
messages to stderr, causing this file to become arbitrarily large.

The disk containing the logfile can be exhausted, possibly causing a
denial-of-service (DoS).